The present invention relates generally to actuator devices and, more particularly, to a single actuator device of multiple active piezoelectric layers.
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, these materials undergo dimensional changes in an applied electric field. The dimensional change (i.e. expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field. Piezoelectric and electrostrictive materials can possess a large number of combined and useful properties such as piezoelectric (electric field dependent strain), electrostrictive, dielectric, pyroelectric (temperature dependent polarization), ferroelectric (electric field dependent polarization) and electrooptic (electric field dependent optical birefringence). These devices have a wide range of application including actuators, switches, pumps, speakers, sensors, switches, hydrophones, hydrospeakers, adaptive optics, variable focus mirrors and lenses, vibrators, benders, accelerometers, strain gauges and saddle inchworms.
Various forms of electroactive devices are known in the prior art. The simplest of such prior devices are the direct mode actuators, such as magnetostrictive actuators and piezo stacks, which make direct use of a change in the dimensions of the material when activated by an electric field, without amplification of the actual displacement. The direct mode actuator includes a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a material which has a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. Although exhibiting high force, these direct mode actuators suffer from the disadvantage of achieving only very small displacement (strain), at best a few tenths of a percent.
Indirect mode actuators provide greater displacement than direct mode actuators. They achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer, such as that disclosed in U.S. Pat. No. 4,999,819, now commonly known as a xe2x80x9cMooniexe2x80x9d. Flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass or similar structure. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit several orders of magnitude greater displacement than that produced by direct mode actuators.
Other examples of indirect bending mode actuators include unimorph and bimorph actuators. One type of unimorph is a structure composed of a single piezoelectric element externally bonded to a flexible metal foil which is stimulated by the piezoelectric element when activated with a changing voltage and results in an axial buckling or deflection as it opposes the movement of the piezoelectric element. The actuator movement for a unimorph can be by contraction or expansion. Unimorphs can exhibit a strain of as high as 10% but can only sustain loads which are less than one pound.
A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than unimorphs because under the applied voltage one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20%, but similar to unimorphs, they cannot sustain loads greater than one pound.
Although the indirect bending mode actuators can exhibit larger strains than direct mode actuators, the load bearing capacity of these devices decreases as the strain increases, resulting in very small load bearing capacities.
U.S. Pat. No. 5,471,721 discloses a pre-stressed unimorph now commonly known as xe2x80x9cRAINBOWxe2x80x9d, which is an acronym for Reduced And INternally Biased Oxide Wafer. In the RAINBOW device, a first surface of a wafer becomes a metallic/conductive reduced form of the ceramic material. In addition, the wafer is concave shaped due to (1) volume shrinkage of the reduced material with respect to the unreduced material and (2) the difference in thermal expansion between the reduced side and the dielectric (unreduced) side. As a result of the concave shape, the reduced side is in tension while the dielectric side is in compression at zero applied field. The net effect is to place the electrically active side (dielectric side) of the RAINBOW wafer in compression, which is a desirable configuration for reliability, increased polarization, load bearing capability and long life. The RAINBOW devices exhibit equal or greater strains and sustain greater loads than bimorphs and conventional unimorphs, but the improvement in load bearing capability is still only moderately better. Furthermore, the chemical reduction process for fabricating RAINBOW actuators releases vapors, such as lead vapors, from the wafer into the atmosphere.
Recently NASA has developed another pre-stressed unimorph now commonly known as xe2x80x9cTHUNDERxe2x80x9d, which is an acronym for THin layer composite UNimorph ferroelectric Driver and sEnsoR. THUNDER is a composite structure constructed with a PZT (lead zirconate titanate) piezoelectric ceramic layer which is electroplated on its two major faces. A metal pre-stress layer is adhered to the electroplated surface on at least one side of the ceramic layer by an adhesive layer, which NASA calls xe2x80x9cLaRC-SI(trademark).xe2x80x9d. During manufacture of the THUNDER actuator, the ceramic layer, the adhesive layer and the first pre-stress layer are simultaneously heated to a temperature above the melting point of the adhesive, and then subsequently allowed to cool, thereby re-solidifying and setting the adhesive layer. During the cooling process the ceramic layer becomes strained, due to the higher coefficients of thermal contraction of the metal pre-stress layer and the adhesive layer than of the ceramic layer. Also, due to the greater thermal contraction of the laminate materials than the ceramic layer the ceramic layer deforms in an arcuate shape having a normally concave face.
The THUNDER device reportedly generates significantly greater output deformation for a given voltage input than do prior ferroelectric and ferrostrictive devices. The THUNDER device, however, still represents only moderate improvement in load bearing capability. NASA does suggest that for certain applications requiring more mechanical output force than can be provided by a single piezoelectric device, two or more devices can be arranged in a xe2x80x9cstacked spoonsxe2x80x9d configuration. A three-THUNDER device arrangement would have a single piezoelectric device discretely bonded to a second single piezoelectric device by a compliant layer, such as an adhesive elastomer, which would then be bonded to a third single piezoelectric device by another compliant layer, thus electrically isolating one device from another. Each single device is independently manufactured prior to stacking with other devices and includes a single active piezoelectric ceramic layer with deposited electrode layers on opposing surfaces, a pre-stressing adhesive layer adjacent one electrode layer and an optional reinforcing material adjacent the adhesive layer. Each ceramic layer has a positive polarity electrode layer and a negative polarity electrode layer associated therewith, such that when stacked, the compliant layer is necessary to insulate un-like poles from each other. Although the stacked THUNDER devices represent higher displacement and force capabilities than prior devices, these devices are only loosely stacked and require a high number of electrodes (2 per device) with insulation therebetween.
It is known that the dome height in thermally pre-stressed bender actuators with thicker laminate layers is less sensitive to changes in temperature than with thinner layers. A thicker ceramic layer, however, requires a higher voltage to achieve the same electric field as a thinner ceramic layer. This represents a further disadvantage of such actuator devices as RAINBOW and THUNDER.
While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as many be included within the spirit and scope of the present invention.
The present invention discloses a piezoelectric actuator, comprising alternating layers of a conductive substrate material and layers of an electrically active ceramic, each alternating layer adhered to the next with an intermediate conductive, thermally-activated adhesive layer to form a bonded laminate, wherein the conductive substrate layers alternate between positive and negative polarity such that the substrate layers are electrodes capable of providing an electric field to each of the ceramic layers.